A typical implementation of a wireless communication device includes a radio transceiver shared by one or more radio access control units (e.g. software stacks) that control operation of the wireless communication device in connection with a respective cellular communication network.
For various reasons, transmission and/or reception gaps may be enforced during the use of the radio transceiver by a radio access control unit, without the network being aware of the gap. For example, if the radio transceiver is shared by two radio access control units, one of the radio access control units may autonomously interrupt the other radio access control unit during use of the radio transceiver (e.g. to perform measurements, listen to paging signals, etc.). Other examples include transmission and/or reception gaps introduced for power saving reasons or because a substantial interference occurs at particular moments in time (e.g. from radar operations of an airport).
During transmission and/or reception gaps, there may be various causes for the network node to increase the robustness of the modulation and coding scheme (MCS) used for downlink transmissions. For example, if a transmission gap occurs, the wireless communication device will not be able to transmit channel condition indication reports (e.g. CQI—channel quality indication—or CSI—channel state information). Alternatively or additionally, the wireless communication device will not be able to transmit HARQ (hybrid automatic repeat request) ACK/NACK (acknowledgement/non-acknowledgement) messages if a transmission gap occurs. Alternatively or additionally, if a reception gap occurs, the wireless communication device will not be able to receive any data, which causes absence of HARQ ACK/NACK. Alternatively or additionally, the wireless communication device will not be able to receive any uplink allocations if a reception gap occurs, which causes absence of uplink data in allocated resources. All of these situations may cause the network to conclude that the channel is bad, and consequently choose a more robust MCS.
However, as soon as the transmission/reception gap is over, the wireless communication device is able to resume communication under the same (or similar) conditions as before the gap. Thus, the more robust MCS may be unnecessarily applied, which causes a decrease in throughput. Since it typically takes some time for the control loop involving channel condition reports from the wireless communicating device and corresponding MCS adjustment to converge, quite a substantial decrease in throughput may be experienced. The decreased throughput may affect the wireless communication device and/or the system as a whole.
The scenario above will be further illustrated below by way of the following examples.
FIG. 1 illustrates a few situations where transmission and/or reception gaps are created autonomously by the wireless communication device.
Part a) illustrates the use 100 of a radio transceiver for UMTS LTE TDD (Universal Mobile Telecommunication Standard, Long Term Evolution, Time Division Duplex), and a corresponding transmission (Tx) blanking 101 (i.e. a transmission gap) during a repetition period 102. Part b) also illustrates the use 110 of a radio transceiver for UMTS LTE TDD, and a corresponding transmission blanking and reception (Rx) puncturing 111 (i.e. a transmission and reception gap) during a repetition period 112.
In part a) and b), the illustrated configuration is UMTS LTE TDD uplink/downlink configuration 1, with Rx puncturing and/or Tx blanking extending over one radio frame.
Part c) illustrates the use 120, 125 of a radio transceiver for uplink (UL) and downlink (DL) UMTS LTE FDD (Frequency Division Duplex), and a corresponding transmission blanking 121 (i.e. a transmission gap) during a repetition period 126. Part d) illustrates the use 130, 135 of a radio transceiver for uplink (UL) and downlink (DL) UMTS LTE FDD, and a corresponding transmission blanking and reception puncturing (i.e. a transmission and reception gap 131) during a repetition period 132.
FIG. 2 illustrates an example with 7 HARQ processes on the downlink (P1, P2, P3, P4, P5, P6, P7 with down-pointing arrows), and corresponding ACK/NACK for LTE TDD uplink/downlink configuration 1 as illustrated by the TDD UL/DL allocation 200. For example, data packets (transport blocks) for DL process P1 is transmitted (or re-transmitted) in sub frame 0 of the first repetition period, and in sub frame 1 of the second repetition period, as indicated by the down-pointing arrows marked with P1. The ACK/NACK of the data packet for DL process P1 transmitted in sub frame 0 of the first repetition period is expected in sub frame 7 of the first repetition period as indicated by the bowed arrow leading from sub frame 0 to sub frame 7. White sub frames indicate downlink reception, dotted sub frames indicate uplink transmission, and striped sub frames indicate special sub frames. If, for example, uplink sub frames 7 and 8 are blanked the network node will not receive any ACK/NACK for the downlink transport blocks of sub frames 0, 1 and 4, even if they may have been successfully received.
As mentioned above, an autonomous gap may cause the network node (e.g. e NodeB—eNB—in UMTS LTE) to assume that the wireless communication device (e.g. user equipment—UE—in UMTS LTE) is experiencing a bad radio channel.
The eNB may assume that the UE has, therefore, failed to decode the downlink control information, which itself is more robust than transmissions on DL-SCH (downlink shared channel).
Alternatively or additionally, the eNB may assume that a previously reported channel quality indication (CQI) is no longer valid. As a consequence, the eNB may back off and assume that the channel quality is lower than indicated by the previously reported CQI.
Alternatively or additionally, the eNB may assume that the particular UE has a bias in its CQI reporting and, for example, reports a more favorable quality than according to the actual channel conditions.
The channel quality indication (CQI) indicates how many information bits can be sent in an allocation of a particular size (e.g. 20 resource block—RB—pairs). At low channel quality (corresponding to low CQI), more error correction encoding and/or a lower order modulation is needed for successful transmission of the information bits, and at high channel quality (corresponding to high CQI), it is possible to use less error correction encoding and/or a higher order modulation and still have successful transmission of the information bits. Hence, the throughput of information bits can be made higher at high CQI than at low CQI, which is illustrated by the example table below (4-bit CQI table of 3GPP TS 36.213 section 7.2.3).
CQI indexmodulationcode rate × 1024efficiency0out of range1QPSK780.15232QPSK1200.23443QPSK1930.37704QPSK3080.60165QPSK4490.87706QPSK6021.1758716QAM3781.4766816QAM4901.9141916QAM6162.40631064QAM4662.73051164QAM5673.32231264QAM6663.90231364QAM7724.52341464QAM8735.11521564QAM9485.5547
When the eNB makes one or more of the assumptions exemplified above due to an autonomous gap it is not aware of, it may choose a more robust MCS (combination of modulation and coding rate), i.e. a MCS corresponding to a lower CQI index, which impacts throughput as exemplified in the efficiency column of the table above.
A few situations where gaps in the use of the radio transceiver by a radio access control unit arise will be described in the following.
Paging
Wireless communication devices (user equipments—UEs) that are idle tune in to the corresponding network node (base station) at predetermined occasions, paging occasions, to check whether they are getting paged by the network. The reason for getting paged may, for instance, be that there is an incoming call for the UE to receive.
While it is in idle mode, the UE is handling the mobility autonomously using neighbor cell information provided by the network. If the current camping cell becomes weak and there is a stronger neighbor cell, the UE will change camping cell to the stronger neighbor. During this—so called—cell reselection, the UE is not monitoring paging and, hence, it may miss if it is getting paged at that moment. To prevent that the paging is missed due to interruption caused by cell reselection, radio access networks are usually repeating the paging one or more times until the UE responds.
All base stations in a so called location (or tracking) area for which the UE has registered are paging the UE. When the UE moves to a cell in another location (or tracking) area, e.g. due to crossing some geographical boundary or changing to another radio access technology, it has to update the network regarding which area it is in via a Location (or Tracking) Area Update procedure. During the time period when the UE is updating the location (or tracking) area, the radio access network will have outdated information regarding the area in which the UE should be paged. To prevent the paging being missed due to outdated location information, the radio access network usually repeats the paging in adjacent location (or tracking) areas if the UE does not respond to paging in the registered location (tracking) area.
Gaps in the use of the radio transceiver by a first radio access control unit may arise if a second radio access control unit needs to listen for pages during a paging occasion.
The paging occasions typically follow a so called paging cycle, which is configured by the radio access network node. The paging cycle length also depends on the radio access technology. Some example idle mode paging cycles include:
GSM—471, 706, 942, 1177, 1412, 1648, 1883, 2118 ms
WCDMA—640, 1280, 2560, 5120 ms
TD-SCDMA—640, 1280, 2560, 5120 ms
LTE—320, 640, 1280, 2560 ms
Circuit-Switched Fallback (CSFB)
Circuit switched fallback is an interim solution for supporting voice calls to UEs that are connected to UMTS LTE until VoLTE (voice over LTE, VoIP) and SRVCC (single radio voice call continuity) are supported in the networks.
This feature allows the UE to be paged in the UMTS LTE system for an incoming call in a legacy system (e.g. a GSM system), and it can then be redirected to the legacy RAT (Radio Access Technology, e.g. GSM). This means that a UE can safely camp on, or be connected to, an UMTS LTE cell without missing any incoming calls.
Typically, the UE gets informed about whether CSFB is supported in the UMTS LTE cell when carrying out a combined registration for CS (circuit switched) and PS (packet switched) services. If CSFB is not supported, the registration will fail. The standard-compliant UE action when CS is not supported is to deactivate the support for UMTS LTE.
If CSFB is not supported, gaps in the use of the radio transceiver by a first radio access control unit (e.g. UMTS LTE) may arise if a second radio access control unit (e.g. GSM) needs to listen for pages to allow UMTS LTE camping or connection while (at the same time) camping on a legacy RAT (e.g. GMS) to monitor CS paging.
Single Radio-LTE (SR-LTE)
In SR-LTE a single radio transceiver is shared between UMTS LTE and a legacy RAT (e.g. GSM) in a time-division manner. The UE is connected to or camping on UMTS LTE while (at the same time) it is camping on a legacy RAT. When, for example, monitoring paging in the legacy RAT, reading system information, carrying out mobility measurements, doing a location area update, or receiving a call in relation to the legacy RAT, the radio transceiver is handed over to the legacy RAT and any UMTS LTE activities may be punctured. A device supporting SR-LTE does not rely on CSFB to allow camping on or being connected to UMTS LTE. SR-LTE can be considered a special case of DSDS (dual SIM dual standby) where both SIMs are from the same operator (physically a single SIM).
Gaps in the use of the radio transceiver by a first radio access control unit (e.g. UMTS LTE) may arise if a second radio access control unit (legacy RAT, e.g. GSM) needs to perform any of the tasks exemplified above.
Monitoring Legacy RAT Using Available Additional Receiver
A UE capable of carrier aggregation may use an available receiver otherwise reserved for a secondary component carrier in carrier aggregation to monitor paging, carry out mobility measurements and/or read system information in the legacy RAT. As long as there is large enough separation between UMTS LTE uplink (UL) and legacy RAT downlink (DL) spectrum, the legacy RAT can be received concurrently with UMTS LTE transmissions on the UL. Hence, for this case the legacy RAT can be monitored without any impact on UMTS LTE performance.
Typically, the problems related to gaps (created by a second radio access control unit) in the use of the radio transceiver by a first radio access control unit do not arise in this case.
If the spectral separation between UMTS LTE UL and legacy RAT DL is not sufficient, collisions between UMTS LTE UL transmissions and legacy RAT reception needs to be avoided in order to prevent high energy leaking from the transmitter to the receiver and destroying the signal to be received, or even destroying the LNA (low-noise amplifier) used in the radio transceiver. In many cases, this will mean that UMTS LTE UI, transmissions need to be skipped when in conflict with legacy RAT activities.
This situation may lead to that the problems related to gaps in the use of the radio transceiver by a first radio access control unit arise.
Dual SIM Dual Standby or Activity
In DSDS (dual SIM dual Standby) and DSDA (dual SIM dual activity) the UE is equipped with two SIM cards, and maintains connectivity (potentially) towards two different networks at the same time (typically for different operators).
For DSDA it is required that the UE uses separate radio transceivers for each connection, since, for example, it may use PS services simultaneously for both SIM identities, or PS service for one SIM and CS service for the other SIM. When one of the connections is terminated but the other still is active, the UE will be in idle mode for the SIM identity associated with the terminated connection. While in idle mode, it will monitor paging and carry out mobility management. For power saving reasons it may be attractive to use only one of the receivers in a time-division manner to maintain connectivity towards a first network and monitor paging in a second network (or for second identity in same network).
Thus, gaps in the use of the radio transceiver by a first (active connection) radio access control unit may arise when a second (idle mode) radio access control unit needs to monitor paging or carry out mobility management.
For DSDS it is not necessary to use two radio transceivers since it is assumed that the UE will be active only towards (at most) one network (or for one SIM identity) at any time, and will only monitor paging and carry out mobility management in the other network. With such a solution, DSDS is essentially similar to SR-LTE in that the radio transceiver is used in a time-division manner with puncturing of the ongoing connection when reading paging from the other network.
Thus, gaps in the use of the radio transceiver by a first radio access control unit (with active connection) may arise if a second radio access control unit (in idle mode) needs to perform any of the tasks exemplified above.
US 2014/0003260 A1 discloses manipulation of modulation and coding scheme (MCS) allocation after a communication interruption. First channel quality information may be generated and transmitted to the base station. A first MCS allocation, which may be based at least in part on the first channel quality information, may be received form the base station. Second channel quality information may be generated and transmitted to the base station, where the second channel quality information is modified by an offset configured to modify a second MCS allocation.
This solution has an inherent delay before an adequate MCS is applied after a communication interruption. Hence a loss of throughput is experienced during the delay. Furthermore, the offset calculation has to be performed every time the method is applied, which is not very resource efficient.
Therefore, there is a need for alternative and improved ways of handling gaps in the use of a radio transceiver.